1. Technical Field
The present invention relates generally to communication networks, and more particularly to resource allocation in a network based on an ARQ protocol.
2. Related Art
Networks prone to relatively high error levels, such as wireless or power line networks, integrate generally an Error Control (EC) unit in the Data Link Layer (LL) in order to retransmit data that have encountered errors. In such systems, a Medium Access Control (MAC) layer is in charge of sharing the resource provided by the PHYsical (PHY) layer between the different users.
Several schemes, which are well known by a person skilled in the art, are based on a centralised Time Division Multiple Access (TDMA) scheme using MAC protocol and MAC Time Frame (MTF). The following sections describe basic principles of this type of scheme.
In such a scheme, STAtions (STAs) that are able to communicate between themselves are grouped together in a cell. A specific entity called Radio Resource Manager (RRM) unit is responsible for distributing the resource provided by the PHY layer among the STAs within the cell. In a TDMA scheme, time is divided in series of MTF, each comprising several reserved time intervals called “dedicated time slot”. The PHY resource allocated to an STA comprises one or more dedicated time slots, in which the STA can transmit data over the transmission medium. Dedicated time slots may have a variable duration and are dynamically granted on a per MTF basis according to the requirements of the STAs. At MAC level, an STA can emit in a dedicated time slot a data traffic received from an upper layer of its protocol stack. In addition, an STA can also transmit signaling messages, generated by different modules comprised in the LL, as for example MAC or EC unit, to a destination STA or to the RRM unit. STAs indicate their resource requirements to the RRM unit through specific MAC signaling messages called Resource Request (RR) messages. According to those requirements, the RRM unit distributes the PHY resource contained within each MTF.
In order to provide an error-free service to the upper layer, an EC unit based on an Automatic Repeat reQuest (ARQ) protocol can be performed to ensure reliable Protocol Data Unit (PDU) transmission. Classically, ARQ protocols rely on an identification of the PDUs that is common to the STA transmitting PDUs and the STA receiving these PDUs, respectively noted a transmitter and a receiver. This identification is used by such a protocol so that the receiver can indicate to the transmitter which PDUs are not correctly received through so called FeedBack Information (FBI) messages. Consequently, upon reception of these FBI messages, the transmitter is able to perform PDUs retransmission. The PDU identification is typically a Sequence Number (SN) specific to each PDU. In general, this SN is handled by a considered layer only.
In order to avoid blocking transmission by waiting for the FBI message each time a PDU is emitted, a sliding window mechanism is generally implemented. In such a sliding window mechanism, the transmitter manages a list of SNs that it is allowed to send, and the receiver manages a list of SNs that it is prepared to receive. Each of these lists is thought of as a window of PDUs. In the transmitter, only the PDUs within such a window can be emitted.
FIG. 1 illustrates a classical ARQ scheme according to the prior art. The system comprises the RRM unit 10. A transmitter ARQ unit 11 and a receiver ARQ unit 12 are respectively associated to a transmitter and a receiver. For the sake of simplification, a single data flow 15 is considered. The direction used to transmit PDUs is called “Forward direction”, while the reverse direction used to return feedback information about the transmitted PDUs is called “Backward direction”.
The transmitter ARQ unit 11 sends an RR message 13 to request resource to the RRM unit 10 for initial PDUs transmission. The transmitter ARQ unit 11 transmits PDUs 15 to the receiver ARQ unit 12 through the resource allocated by the RRM unit 10 to it. On the other hand, the receiver ARQ unit sends a RR message 16 to the RRM unit 10 to request resource for FBI messages transmission. Upon reception of the transmitted PDUs, the receiver ARQ unit sends an FBI message 14 through the resource allocated by the RRM unit to it.
In error-prone systems, such as wireless or power line networks, as already described above, an EC unit is introduced in the MAC layer and an ARQ scheme is often implemented. As result of the above comments to FIG. 1, such an ARQ scheme induces a signaling overhead. Actually, both the transmitter ARQ unit and the receiver ARQ unit use RR messages to request resource from the RRM unit, in order to send PDUs or FBI messages respectively. As a result of this additional signaling overhead, latency and extra bandwidth consumption are introduced.
It has been proposed to limit resource usage and transfer latency by implementing a Selective ARQ (SARQ) scheme. According to such a scheme, the incorrectly received PDUs are individually advertised by the receiver and retransmitted by the transmitter. Such SARQ scheme minimizes the PDU retransmission because only not correctly received PDUs are retransmitted, instead of all PDUs comprised in the transmitter ARQ sliding window.
However, minimizing the bandwidth required for PDUs retransmission may not reduce the amount of resource used for signaling messages in order to perform such a SARQ protocol. Moreover, the transfer delay can even be higher.
It results from the foregoing that it is difficult to apply an efficient resource allocation in a network based on an ARQ protocol.